This invention relates generally to air motors and more particularly to a piston to crank linkage for an air motor for a hoist.
In a reciprocating piston air motor the usual method of transferring the force of each piston to the crank shaft is through a connecting rod, one end of which pivots on a wrist pin in the piston, the other end being connected by a bearing assembly to the crank shaft. This arrangement is called a linked piston motor.
Linked piston air motors utilize one of two well known bearing arrangements for connecting the piston rods to the crank shaft. In both of these arrangements the crank shaft has a single journal to which the rods are connected at the same axial location or, in the case of six piston motors, at two adjacent axial locations.
In the common bearing arrangement the rod ends are segmented so that together they form a complete bearing that encircles the crankshaft journal. The segmented rod ends have sufficient clearance between adjacent rod ends to allow the rods to pivot independently and are held against the journal by retaining rings that encircle the outside of the segments.
In the second bearing arrangement one of the connecting rods, known as the "Queen Rod" contains a bearing that fits over the crankshaft journal. The bearing housing on this rod has a flange around the outside to which the remainder of the rods are pinned.
In order to accommodate the frictional resistance from side loading of the connecting rods, the pistons of these motors require sufficient skirt length to prevent them from tipping and jamming in the cylinder bore. The ratio of piston length to diameter is usually greater than 0.6, adding significantly to the size and inertia of the moving parts. This results in an air motor that is large in comparison to its power output. Because of the requirement to pin the rods outside of the crank bearing, the "Queen Rod" air motor requires even greater space.
A well known alternative that overcomes the size and inertia problems of the linked piston motor is the "Scotch Yoke" piston motor. In this motor, pairs of opposing pistons are yoked together and their force is transferred to a bearing on the crank that rides back and forth in a slot in the yoke. With the pistons yoked together their effective bearing length is increased to the distance across the motor between one opposing piston and the other. This allows the pistons to be very short in comparison to their diameter, their length need only be great enough to accommodate a sealing ring that can also be designed to handle the side loads. The method of transferring the piston force through a bearing riding in a slot in the yoke requires the bearing to reverse rotation every 90 degrees of crank rotation. For this reason, the yoke must be hardened steel to withstand the wear caused by the bearing scuffing at high speeds. An additional bearing arrangement is usually required to prevent the yoke from twisting on its axis which would throw it out of perpendicular alignment with the crank pin.
With the exception of the "Queen Rod" arrangement, these motors require oil bath lubrication in the crankcase to prevent wear of the crank bearings and remove frictional heat. The "Queen Rod" motor could be designed to utilize sealed anti-friction bearings and therefore avoid the need for oil bath lubrication but this would also increase overall size.
The present invention combines the principles of the "Queen Rod" and "Scotch Yoke" arrangements in a novel way to provide a piston and crank linkage that is light and compact and does not require oil bath lubrication.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.